The present invention relates to optical sheets, for example films, suitable for use in spreading light.
A problem that is frequently encountered when an area is illuminated using artificial light sources or natural daylight is how to spread the light adequately and evenly. In the case, for example, in which the area that is being illuminated is a floor area within a building, there will usually be parts of the floor area that are less well lit than others, and also some locations where the users of the building are troubled by glare from the light source.
Simple diffusers in the form of a sheet of translucent material are often used to spread light but, although they are inexpensive, they do not transmit light efficiently and are also unable to direct light in a controlled manner to where it is needed.
U.S. Pat. No. 5,551,042 describes a structured optical film for use with skylights or dormer windows to provide improved illumination of the interior of buildings with natural daylight. The film is intended to be positioned at a window with the structured surface (which forms a plurality of negative Fresnel lenses) presented towards the sunlight and enables a reasonably uniform, non-glare illumination of a floor area of the building to be achieved.
Other structures, in the form of prism glass plates, for distributing daylight are described in U.S. Pat. Nos. 28,019, 586,220, 720,386 and 818,208.
Structured films and sheet materials have also been described for use in luminaires, to distribute the light from artificial sources. U.S. Pat. Nos. 5,150,966, 5,029,060 and 4,755,921, for example, describe the use of catadioptric Fresnel lenses in light fixtures: in each case, the lens is formed from a sheet of transparent material having a structured surface comprising prism structures. U.S. Pat. No. 1,612,804 and GB-A-762 769 describe luminaires with refractors that, on the surface remote from the light source, have a plurality of prisms for distributing the light in certain directions.
In the field of backlit optical displays, for example the liquid crystal displays used in portable personal computers, calculators etc., it is also known to use structured optical sheet materials to redirect the light passing through the display with a view to increasing the amount of light emitted in a direction approximately normal to the display. Examples of sheet materials intended for that purpose are described in U.S. Pat. Nos. 5,467,208 and 5,771,328.
The present invention is concerned with the problem of spreading light in a controlled manner applicable to both natural and artificial light and, in the case of the latter, applicable not only to traditional incandescent and fluorescent light sources but also to more recently-developed light sources such as light emitting diodes (LEDs).
The present invention provides an optical sheet suitable for spreading light, in which a first major surface of the sheet is a structured surface comprising an array of prisms such that, if the second major surface of the film is substantially smooth, a normally-incident beam of light that enters the film through the second major surface and leaves through the array of prisms will be spread by the latter; wherein the array comprises:
(a) a plurality of reflection prisms selected to deviate the normally-incident light through different angles and a plurality of refraction prisms selected to deviate the normally-incident light through different angles, in which successive reflection prisms are separated by at least one refraction prism; or
(b) a non-ordered arrangement of a plurality of reflection prisms selected to deviate the normally-incident light through different angles; or
(c) a non-ordered arrangement of a plurality of refraction prisms selected to deviate the normally-incident light through different angles; in which each refraction prism deviates the normally-incident light only by refraction at a prism facet as the light leaves the film through the first major surface and each reflection prism deviates the normally-incident light by total internal reflection within the prism before the light leaves the film through the first major surface.
The present invention also provides an optical sheet suitable for spreading light, in which a first major surface of the sheet is a structured surface comprising an array of prisms and the second major surface of the sheet is substantially smooth, whereby a normally-incident beam of light that enters the film through the second major surface and leaves through the array of prisms will be spread by the latter;
wherein the array comprises:
a non-ordered arrangement of a plurality of reflection prisms selected to deviate the normally-incident light through different angles and a plurality of refraction prisms selected to deviate the normally-incident light through different angles, in which successive reflection prisms are separated by at least one refraction prism;
in which each refraction prism deviates the normally-incident light only by refraction at a prism facet as the light leaves the film through the first major surface and each reflection prism deviates the normally-incident light by total internal reflection within the prism before the light leaves the film through the first major surface.
Typically, the structured surface of an optical sheet in accordance with the invention comprises a multiplicity of reflection and/or refraction prisms selected to deviate the normally-incident light through respective angles.
The term xe2x80x9clightxe2x80x9d as used herein refers to electromagnetic radiation in the ultraviolet, visible and/or infra-red regions of the electromagnetic spectrum.
The term xe2x80x9cprismxe2x80x9d as used herein normally refers to a structure whose two ends are similar, equal and parallel rectilinear figures, and whose sides are parallelograms. In its simplest form, a prism has a triangular cross-section. However, as used herein, the term extends to structures having cross-sections with more than three sides and also to the limiting case in which the structure has a cross-section with a multiplicity of sides to the extent that at least some of those sides form a curve. In the case of refraction prisms, the term as used herein also includes the limiting case of a triangular prism with a vertex angle of 180xc2x0 (equivalent to the absence of a prism).
As used herein, the term xe2x80x9cnon-orderedxe2x80x9d when applied to a plurality of prisms that deviate light through different angles means that there is no discernible order or pattern in the manner in which those prisms are arranged.